This invention relates generally to implants for repairing ducts and passageways in the body. More specifically, the invention relates to an expandable stent-graft.
Treatment or isolation of vascular aneurysms or of vessel walls which have been thinned or thickened by disease has traditionally been performed via surgical bypassing with vascular grafts. Shortcomings of this procedure include the morbidity and mortality associated with surgery, long recovery times after surgery, and the high incidence of repeat intervention needed due to limitations of the graft or of the procedure.
Vessels thickened by disease are currently sometimes treated less invasively with intraluminal stents that mechanically hold these vessels open either subsequent to or as an adjunct to a balloon angioplasty procedure. Shortcomings of current stents include the use of highly thrombogenic materials (stainless steels, tantalum, ELGILOY) which are exposed to blood, the general failure of these materials to attract and support functional endothelium, the irregular stent/vessel surface that causes unnatural blood flow patterns, and the mismatch of mechanical compliance and flexibility between the vessel and the stent.
Various attempts have been made to provide a nonthrombogenic blood-carrying conduit. Pinchuk, in U.S. Pat. Nos. 5,019,090, 5,092,887, and 5,163,958, suggests a spring stent which appears to circumferentially and helically wind about as it is finally deployed except, perhaps, at the very end link of the stent. The Pinchuk ""958 patent further suggests the use of a pyrolytic carbon layer on the surface of the stent to present a porous surface of improved antithrombogenic properties.
U.S. Pat. No. 5,123,917, to Lee, suggests an expandable vascular graft having a flexible cylindrical inner tubing and a number of xe2x80x9cscaffold membersxe2x80x9d which are expandable, ring-like and provide circumferential rigidity to the graft. The scaffold members are deployed by deforming them beyond their plastic limit using, e.g., an angioplasty balloon.
A variety of stent-graft designs also have been developed to improve upon simple stent configurations. Perhaps the most widely known stent-graft is shown in Ersek, U.S. Pat. No. 3,657,744. Ersek shows a system for deploying expandable, plastically deformable stents of metal mesh having an attached graft through the use of an expansion tool.
Palmaz describes a variety of expandable intraluminal vascular grafts in a sequence of patents: U.S. Pat. Nos. 4,733,665; 4,739,762; 4,776,337; and 5,102,417. The Palmaz ""665 patent suggests grafts (which also function as stents) that are expanded using angioplasty balloons. The grafts are variously a wire mesh tube or of a plurality of thin bars fixedly secured to each other. The devices are installed, e.g., using an angioplasty balloon and consequently are not seen to be self-expanding. The Palmaz ""762 and ""337 patents appear to suggest the use of thin-walled, biologically inert materials on the outer periphery of the earlier-described stents. Finally, the Palmaz ""417 patent describes the use of multiple stent sections each flexibly connected to its neighbor.
Rhodes, U.S. Pat No. 5,122,154, shows an expandable stent-graft made to be expanded using a balloon catheter. The stent is a sequence of ring-like members formed of links spaced apart along the graft. The graft is a sleeve of a material such as an expanded polyfluorocarbon, expanded polytetrafluoroethylene available from W. L. Gore and Associates, Inc. or IMPRA Corporation.
Schatz, U.S. Pat. No. 5,195,984, shows an expandable intraluminal stent and graft related in concept to the Palmaz patents discussed above. Schatz discusses, in addition, the use of flexibly-connecting vascular grafts which contain several of the Palmaz stent rings to allow flexibility of the overall structure in following curving body lumen.
Cragg, xe2x80x9cPercutaneous Femoropopliteal Graft Placementxe2x80x9d, Radiology, vol. 187, no. 3, pp. 643-648 (1993), shows a stent-graft of a self-expanding, nitinol, zig-zag, helically wound stent having a section of polytetrafluoroethylene tubing sewed to the interior of the stent.
Cragg (European Patent Application 0,556,850) discloses an intraluminal stent made up of a continuous helix of zig-zag wire and having loops at each apex of the zig-zags. Those loops on the adjacent apexes are individually tied together to form diamond-shaped openings among the wires. The stent may be made of a metal such as nitinol (col. 3 lines 15-25 and col. 4, lines 42+), and may be associated with a xe2x80x9cpolytetrafluoroethylene (PTFE), dacron, or any other suitable biocompatible materialxe2x80x9d. Those biocompatible materials may be inside the stent (col. 3 lines 52+) or outside the stent (col. 4, lines 6+).
WO93/13825 to Maeda et al. discloses a self-expanding stent having a wire bent into an elongated zig-zag pattern and helically would about a tubular shape interconnected with a filament. A sleeve may be attached to the outer or inner surface of the stent.
PCT application publication WO/95/05132 discloses a stent-graft with a tubular diametrically adjustable stent.
There is a need for an alternate stent-graft construction that exhibits excellent kink resistance and flexibility.
The present invention involves a stent-graft including a stent member having an inner surface and an outer surface, a generally tubular graft member and a coupling member that couples the stent member to the graft member. The coupling member, which in the preferred embodiment is in the form of a ribbon, covers only a portion of at least one of the inner or outer surface of the stent member and secures the stent member and graft member to one another. Alternatively, the coupling member can be described as interconnecting less than entirely the inner or outer surface of the stent member to the graft member.
With this construction, regions of the stent member do not interface with the coupling member. This is believed to advantageously reduce shear stresses between the stent member and the coupling member when the stent-graft undergoes bending so that tearing of the coupling and/or graft member can be minimized or eliminated. It is also believed that this arrangement minimizes the likelihood of delamination between the coupling member and the graft. If delamination were to occur, the inner portion of the stent-graft could perceivable collapse into the vessel lumen and interfere with desired blood flow. Thus, the stent-graft is believed to be capable of conforming to curves in a blood vessel lumen with minimal risk of tearing the graft or coupling member, or delamination between the stent and graft members.
According to another aspect of the invention, the coupling member is secured to the graft member without sutures. When the graft member is placed within the stent member, for example, this arrangement eliminates the need for having sutures extend into the lumen formed by the graft member and possibly interfere with blood flow. Another benefit of this arrangement, as compared to suturing the stent to the graft member, is that suture holes need not be placed in the graft which could adversely affect its integrity. The coupling member may be thermally or adhesively bonded to the graft member.
The coupling member preferably has a generally broad or flat working surface as compared to filament or thread-like structures such as sutures. As noted above, a preferred coupling member is in the form of a ribbon. This configuration advantageously increases potential bonding surface area between the coupling member and the graft member to enhance the integrity of the bond therebetween. The increased bonding surface may facilitate minimizing the thickness of the coupling member so that the stent-graft lumen volume and blood flow dynamics therein can be optimized. For example, a thicker coupling member would increase the overall stent-graft thickness which can cause an undesirable lumen diameter reduction at the transition where the vessel lumen interfaces the inlet of the stent-graft. This, in turn, can result in undesirable turbulent flow which possibly can lead to complications such as thrombosis.
According to a preferred embodiment of the invention, the coupling member is arranged in a helical configuration with multiple turns. Each of a number of the coupling member turns is spaced from the turn(s) adjacent thereto. With this construction, a generally uniform distribution of coupling member-free stress relief zones may be achieved. Elastic wrinkling in the graft member may occur in those zones so that the graft member can absorb stress when bent along its longitudinal axis, for example, and resist kinking.
According to a preferred stent member construction for use with the stent-graft of the present invention, at least a portion of the stent member includes undulations and is arranged in a helical configuration with multiple turns. Each stent member undulation includes an apex and an open base portion. The apexes and base portions are configured so as not to restrain one apex into the undulation in an adjacent turn and substantially in-phase therewith when the stent-graft is bent or compressed. This is believed to facilitate undulation movement during bending or compression and minimize the likelihood of stress build-up that may cause kinking. The coupling member typically covers a substantial portion of each undulation so as to minimize the likelihood of the stent member apexes bending away from the graft member and interfering with the environment or tether line which may be used to maintain the stent-graft in a folded state before deployment. The coupling member also may be positioned adjacent to the apexes to minimize the likelihood of such apex movement.
According to another aspect of the invention, the end portions of the stent-member also may be enveloped between the coupling member or discrete coupling members and the graft member. This prevents the terminal portions of the stent and graft members from significantly moving away from one another. For example, when the stent-member is external to the graft member, the terminal graft portions may flap away from the stent member and possibly interfere with blood flow if the terminal coupling portions were not present.
According to another feature of the invention, the stent-graft is advantageously manufactured by placing a cushioning layer around a mandrel, assembling the stent-graft on the cushioning layer, surrounding the mandrel mounted assembly with a multi-component member formed from a PTFE tube having a longitudinal slit and which is wrapped with an expanded PTFE or other film or tape to compress the assembly, and heating the assembly to bond a coupling member to the graft.
The above is a brief description of some deficiencies in the prior art, and advantages and aspects of the present invention. Other features, advantages, and embodiments of the invention will be apparent to those skilled in the art from the following description, accompanying drawings and appended claims.